To get classic Outlook working again and reinstall it on Windows, use the official classic Outlook installer instead of relying on the “go back to classic” button or Store-only options.
From classic Outlook, turn on Try the new Outlook once so both apps are available. Then toggle back to classic as above. Pin both Outlook (classic) and new Outlook icons to the taskbar or Start for easy access. Mail, calendar, and folders in Exchange/Outlook.com accounts remain intact; reverting only changes which client is used to access them.
I understand that you'd like to go back to the old version of Outlook or what we now call the classic version of Outlook for Windows. Assuming that you already are using "new" Outlook for Windows app now, you can revert this change by using the steps from the guide below.
To switch from the new Outlook for Windows back to the classic version, follow these steps: Open the New Outlook: Launch the new Outlook application on your Windows device. Locate the Toggle: Look for the toggle option, which is usually found in the top right corner of the Outlook window.
When the dialog appears, choose: Switch back to classic Outlook – this changes the default back to classic Outlook. The next time classic Outlook is launched, it will open the classic app instead of redirecting to new Outlook. Use classic Outlook this time – this opens classic Outlook once, but keeps new Outlook as the default for future ...
Try to launch Classic Outlook Directly Using the EXE. Sometimes the New Outlook hijacks the shortcuts. Try launching classic Outlook manually: Press Windows + R Paste this path and press Enter: ... or for older installs: ... If Classic Outlook opens normally, re-pin it to the taskbar and stop using the New Outlook shortcut. See if this helps.